There is an uncertainty about the sound pressure produced by a hearing instrument when located at or in an individual user's ear. The uncertainty arises from the a priori unknown individual ear characteristics. Individual ears can differ in the geometrical shape and volume of the ear canal and the properties of the tympanic membrane. These factors influence the acoustical behaviour of the ear when it is stimulated by a hearing instrument.
Current solutions to decrease this uncertainty are to measure the individual ear's characteristics prior to or during a hearing aid fitting with external measuring equipment. The first approach uses the so-called real-ear-to-coupler difference (RECD) as a measure of how an individual ear differs from a standard ear, e.g. represented by a standard 2 cc-coupler. This difference is then accounted for during the fitting of a hearing instrument. The second approach uses real time monitoring of the sound pressure in the individual ear when the hearing instrument is inserted into the ear (real ear measurements, REM). The monitoring is e.g. done via a small probe tube inserted into the ear and connected to a microphone of the external measuring equipment.
Both approaches use additional measuring equipment and require additional, time—consuming steps to be performed during a hearing instrument fitting. In addition, they suffer from translational errors because the measurement conditions do not fully correspond to real wearing conditions. In the RECD approach, it is assumed that the hearing aid behaves the same way as the measurement transducer used during RECD measurement, and in the REM approach, the probe tube creates acoustical leakage not present in real wearing conditions.
Further, when placing a hearing device comprising a loudspeaker (receiver) in the ear (RITE), the placement in the ear canal of the loudspeaker can vary from time to time, and may therefore create different resonances in the audio band. This will create a “different” acoustic fitting each time the hearing device is mounted in the ear.
US2007036377A1 describes a hearing instrument comprising at least one inner microphone operable to determine a sensing signal representative of an acoustic signal at a position in front of the user's eardrum. The inner microphone creates a sensing signal representative of the acoustic signal, and the signal processing unit of the hearing instrument determines a characteristic of the user's ear canal based thereon and memorizes values indicative of the characteristic. According to a preferred embodiment, the characteristic is an acoustic coupling transfer characteristic, which is determined based on a comparison of a signal representative of the output signal of the signal processing unit's digital signal processing stage and the sensing signal.
EP2039216B1 relates to a method for monitoring a hearing device comprising an electroacoustic output transducer worn at a user's ear or in a user's ear canal, the method comprising: measuring the electrical impedance of the output transducer analyzing the measured electrical impedance of the output transducer in order to evaluate the status of the output transducer and/or of an acoustical system cooperating with the output transducer and outputting a status signal representative of the status of the output transducer and/or of the acoustical system cooperating with the output transducer.